Inorg. Chem. 1983, 22, 2851-2858
2851
Contribution from the Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138, and the Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Electronic Properties o€ Single- and Double-MoFe3S4Cubane-Type Clusters P. K. MASCHARAK,Ia G. C. PAPAEFTHYMIOU,IbW. H. ARMSTRONG,lBS. FONER,lb R. B. FRANKEL,Ib and R. H. HOLM*Ia
Received December 28, 1982
Electronic properties of the single-cubane clusters [MoFe3S4(SR),(3,6-R'zcat)(solv)]z-~~ and [MOF~~S,(SR)~(~,~-R'~C~~)]~and the double-cubane cluster [Mo2Fe6S8(SR)6(3,6-R'zcat)2]4 (3,6-R'2cat = 3,6-diallyl- or 3,6-di-n-propylcatecholate, soh = solvent) have been investigated by Mossbauer and EPR spectroscopy and magnetization measurements. In most cases R = pC6H4CIand R' = allyl. It is shown that oxidized ([MoFe,S4l3+)single cubanes consist of electronically delocalized, single cubane are antiparallel spin-coupled clusters with S = 3/2 ground states. Properties of one reduced ([MoFeSS4]2+) similar, and its ambient-temperaturemagnetic moment indicates a S = 2 ground state. The isomer shift difference between the oxidized and reduced clusters reveals that the Fe, portions are those primarily affected by electron gain or loss. The bridged double cubane, containing two [MoFe3S4I3+subclusters linked by two Mo-(SR)-Fe bridges, exhibits subcluster spin coupling resulting in a singlet ground state. Low-temperaturemagnetization data for the triply bridged double cubane [M02Fe6S8(SR)g]3-reveals much weaker subcluster interaction and a quartet ground state for each subcluster. Isomer shifts, quadrupole splittings, and hyperfine coupling constants are presented as well as a typical single-cubane EPR spectrum. Current interpretations of the Miissbauer, NMR, and EPR spectral properties of MoFe3S4clusters are summarized. Comparison of certain electronic properties with those of the native S = 3/2 Mo-FeS cluster of nitrogenase reveals a number of similarities. This information, together with EXAFS and X-ray structural data, demonstrates that some electronic and geometrical features of MoFe3S4clusters approach those of the native cluster. These results encourage further synthetic attempts directed toward attainment of Mo-Fe-S clusters whose Fe:Mo atom ratios are closer to that ((6-8):l) of the native cluster. Introduction the terminal reduced member 3 has been isolated and structurally ~haracterized.'~In this series electrons are added In the rapidly expanding area of heteronuclear MFe3S4 successively to individual subcl~sters,'~ resulting in reduction cubane-type clusters (M = Mo, W) two general structural of the initial core oxidation level (Y = [MoFe3S4l3+to j3 = categories, comprised of species having two variously bridged [MOF~,S,]~+. Consistent with this behavior is the reversible subclusters or a single cluster, have emerged. Double cubanes include triply bridged [M2Fe6S8(p-S)(p-SR)2(SR)6]3-,2-4one-electron reduction of the solvated single cubanes 5 to 7l2I6 (reaction 5 ) in the same potential interval (ca. -1.0 to -1.5 [ M2Fe&(p-sR),(sR)6] 3- (1),2-6and [ M2Fe&(p-SR),V vs. SCE) as that for the traversal of series 4. (SR)6]3-,4-3*7and doubly bridged [M2Fe6S8(p-SR)2(SR)4The clusters 1 and their methoxide-bridge variants (3,6-R',cat),]" (4; R' = allyl, n-Pr, cat = catecholate).s-10 [M2Fe6s8(p-oMe),(sR)6]3-, being among the first Single cubanes are of two types, the solvated clusters MFe3S4-type species synthesized,2,6have been the objects of [MFe3S4(SR)3(3,6-R'2cat)(solv)]2(5)8-'0 and the ligated detailed electronic structural studies. Extensive clusters [MFe3S4(SR)3(3,6-R'2cat)L]2-+* (6).8J@13 Reactions M O ~ s b a u e r , ~ NMR,3,6 J ~ J ~ and EPR17 spectral and magnetic 1-3, which afford 5-7 from the precursor double cubane 4, su~ceptibi1ity'~J~ data have been collected and analyzed. Imare shown in Figure 1 together with schematic structural portant electronic structural properties addressed in these formulas for clusters 1-7. investigations include ground spin states, spin coupling within In addition to clusters produced directly by synthesis, and between bridged subclusters, metal site charge (oxidation one-electron redox products that retain the structural elements state) distributions, electron delocalization between (Y and j3 of the oxidized forms can be obtained by chemical or elecoxidation levels, and dominant isotropic interactions in N M R trochemical means. Thus the triply bridged trianion 1 can be spectra. In the present work the Mossbauer, EPR, and reduced to 2 and 3 in chemically reversible one-electron steps, magnetization properties of the more recently synthesized forming the electron-transfer series 4.3J4J5 An example of clusters 4,6, and 7 (M = Mo) have been investigated in order to clarify certain of the preceding properties. Because of the (1) (a) Harvard University. (b) MIT. different bridge structures in 4 vs. 1 and the absence of an (2) Wolff, T. E.;Berg, J. M.; Hcdgson, K. 0.;Frankel, R. B.; Holm, R. H. J . Am. Chem. SOC.1979,101,4140. appended, potentially perturbing second cluster in the single (3) Wolff, T.E.;Power, P. P.; Frankel, R. B.; Holm, R. H. J . Am. Chem. cubanes 6 and 7, significant differences in certain electronic Soc. 1980,102,4694. properties are anticipated and have been found. This inves(4) Palermo, R. E.; Power, P. P.; Holm, R. H. Inorg. Chem. 1982,2I,173. (5) Christou, G.;Garner, C. D.; Mabbs, F. E.; King, T.J. J . Chem. SOC., tigation forms part of our continuing development of MoFe3S4 Chem. Commun. 1978, 740. Acott, S. R.;Christou, G.; Gamer, C. D.; clusters as initial structural and electronic models of the FeMo King, T. J.; Mabbs, F. E.; Miller, R. M. Inorg. Chim. Acta 1979,35, cofactor (FeMo-co) of nitrogenase. Our approach to a synL337. Christou, G.;Garner, C. D.; Miller, R. M.; King, T. J. J . Inorg. thetic representation of FeMo-co and many of the results Biochem. 1979,11, 349. obtained thus far have been reviewed e l s e ~ h e r e . * ~ J ~ (6) Christou, G.;Garner, C. D. J. Chem. SOC.,Dalton Trans. 1980,2354. (7) Wolff, T.E.;Berg, J. M.; Power, P. P.; Hodgson, K. 0.;Holm, R. H. Inorg. Chem. 1980, 19, 430. Experimental Section (8) Armstrong, W. H.; Holm, R. H. J . Am. Chem. SOC.1981,103,6246. Preperation of Compounds. The following compounds were obtained (9) . . Armstrong, W. H.; Mascharak. P. K.; Holm, R. H. J. Am. Chem. Soc. by published methods: (Et4N),[M02Fe6S8(S-p-c6H4Cl)9] 1982,104;4373. (10) Mascharak, P. K.;Smith, M. C.; Armstrong, W. H.; Burgess, B. K.; Holm. R. H. Proc. Natl. Acad. Sci. U.S.A.1982. 79. 7056. (1 1) Armstrong, W.H.; Mascharak, P. K.; Holm, R. H: I d r g . Chem. 1982, 21, 1699. (12) Mascharak, P.K.;Armstrong, W. H. Mizobe, Y.; Holm, R. H. J . Am. Chem. SOC.1983,105, 475. (13) Palermo, R. E.;Holm, R. H. J . Am. Chem. SOC.1983, 105, 4310. (14) Christou, G.; Mascharak, P. K.; Armstrong, W. H.; Papaefthymiou, G. C.; Frankel, R. B.; Holm, R. H. J. Am. Chem. SOC.1982,104,2820. (15) Christou, G.;Garner, C. D.; Miller, R. M.; Johnson, C. E.; Rush, J. D. J . Chem. Soc., Dalton Trans. 1980,2363.
0020-1669/83/1322-2851$01.50/0
(Et4N)4[MozFe6S8(SEt)6(pr2Cat)2],9
(E~~N)~[MO~F~~SS(S-~-
C6H4C1)6((al)Zcat)21(4);' (Et4N)3[MFe3s4(s-pC6H4C1)3((al)zcat)Ll (16) Mizobe, Y.; Mascharak, P. K.; Palermo, R. E.; Holm, R. H. Inorg. Chim. Acta, in press. (17) Christou, G.; Collison, D.; Garner, C. D.; Acott, S. R.; Mabbs, F. E.; Petrouleas, V. J . Chem. SOC.,Dalton Trans. 1982,1575. (18) Holm, R. H. Chem. Soc. Reu. 1981,10,455. (19) Holm, R. H.;Armstrong, W. H.; Christou, G.; Mascharak, P. K.; Mizobe, Y.; Palermo, R. E.; Yamamura, T.In 'Biomimetic Chemistry"; Yoshida, Z., Ise, N., Eds.; Elsevier: New York, 1983;pp 79-99.
0 1983 American Chemical Society
2852 Inorganic Chemistry, Vol. 22, No. 20, 1983
Mascharak et al.
Table I. Magnetic and Mossbauer Spectral Data for Mo-F6S Clusters compd peff,a p B (Et,N), [MoFe,S,(S-p-C, H,Cl),((al),cat)l (6)
-
~ , mm/s b
T,K 120
solid
4.08
80
MeCN soln
4.03
130 80
(Et,N), [MoFe,S,(S-p-C,H,C1),((al),cat)(EtCN)I solid
5.12
4.2
4.99
...
4.2
At -297 K;peff = 2.828(x~T‘)~’~p.e. 20.03 mm/s; relative to Fe metal at 4.2 K.
~
0.30 0.30 0.31 0.28 0.3 1 0.31 0.32 0.32
1.18 1.60 1.14 1.76 0.81 1.24 0.94 1.30
0.46 0.52 0.46 0.46 0.40 0.40 0.40 0.40
64 36 66 34 49 51 44 56
0.42 0.40
0.91 2.25
0.40 0.37
68 32
...
...
...
...
0.28 0.34
1.41 1.10
0.26 0.28
62 38
k0.05 mm/s.
tive intensity (area) of quadrupole doublet.
SR
%
e
~
(7)
MeCN soln (Et,N), [Mo,Fe6S8(S-p-C,H,CI),((al),cat), I (4) solid
RS
~~
MQF mm/s-r,d mm/s
Line width at half-height.
e
Rela-
3-,4-,5-
* t
-
v1
z 0.98 W
t-
z 0.96-400
sR
4 -
2-
5
i
// SR
(3)NO+C,,H;
2; 3-
SR
L
3-
iolv
6
7
F i i 1. Schematic structural formulas of triply bridged (1-3),doubly bridged (4), and single cubanes ( 5 7 ) and reactions 1-3, which lead to the formation of 5-7 from 4.
= +e-
7 +e-
[ M O ~ F ~ ~ S ~ ( S R ) ~[ M ] ~O- ~ F ~ ~ S ~ ( S R ) ~ ] ~ 1 (2a) -e2 ( a p)
+
-
3
[MoFe3S4(SR),(3,6-R’,cat) (solv)] *-e5 (a) [MoFe,S,( SR),( 3,6-R’,cat) (solv)]3- (5)
7 (PI ( 6 L = pCIC6H4S-,M = Mo; L = CN, M = Mo, W).I2 Catecholate ligands are designated by Pr2cat (3,6-di-n-propylcatecholate)and (al)pt (3,6-diallylcatecholate). The solvated clusters 5 were generated in solution by reaction 1. The reduced solvated cluster salt (7, solv = EtCN) (Et4N)J MOF~,S~(S-~-C,H~CI)~((~~)~C~~)(E~CN)] was prepared by reaction 3; details of the synthesis are reported elsewhere.’, Physical Measurements. Magnetic susceptibilities of solid and
-200 000 ZOO VELOCITY (mmlsec)
400
Mossbauer spectrum of polycrystalline (Et,N)![M~Fe~S~(S-p-C,H~Cl)~((al)~cat)] (6) at 120 K. The solid line is a theoretical least-squares fit to the data assuming two inequivalent Fe sites and based on the parameters in Table I. Figure 2.
solution samples at ambient temperature were determined by the Faraday (H~CO(NCS)~ calibrant) and ‘H NMR” (Me4Sireference) methods, respectively. Solvent susceptibility2I and diamagnetic corrections22were taken from published data. EPR spectra were determined at X-band frequencies with a Varian E-109 spectrometer equipped with a Helitran Model LTD-3-110temperature controller. Mossbauer spectral measurements were made with a conventional constant-acceleration spectrometer equipped with a ”Co source in a rhodium matrix maintained at the same temperatures as the absorber. Spectra were measured in zero applied magnetic field at various temperatures and in longitudinallyapplied fields up to 80 kOe at 4.2 K. Polycrystalline and solution samples were prepared as previously described.,’ Cluster concentrations in solution samples were 30 mM. Magnetization measurements of solid samples were carried out at 1.43 and 4.2 K over an external magnetic field range of 0-90 kOe with a vibrating-sample magnetometeS4adapted to a superconducting solenoid. Samples of ca. 20-30 mg were measured in thin-walled Delrin or Kel-F containers. All measurementswere performed under strictly anaerobic conditions.
-
Results and Discussion Electronic properties of the single cubanes 6 and 7 and the doubly bridged double cubane 4 as manifested in S7Fe Mossbauer spectra and low-temperature magnetization behavior are considered in the following sections. Mossbauer parameters based on two-site fits of experimental spectra and magnetic moments a t ambient temperature are collected in Table I. All isomer shifts are referenced to Fe metal a t the same temperature as the absorber. [M~Fe,S,(S-p-C~H,Cl)~((al)~cat)]~(6). The Mossbauer (20) Phillips, W. D.; Poe, M. Methods Enzymol. 1972, 24, 304. (21) Gerger, W.; Mayer, U.; Gutmann, V. Monatsh. Chem. 1977, 417. (22) Mulay, L. N. In “Physical Methods of Chemistry”; Weissberger, A., Rossiter, B. W., Eds.;Wiley-Interscience: New York, 1972; Part IV, Chapter VII. (23) Laskowski, E. J.; Reynolds, J. G.; Frankel, R. B.; Foner, S.;Papaefthymiou, G. C.; Holm, R. H.J . Am. Chem. SOC.1979, 101, 6562. (24) Foner, S . Rev. Sci. Instrum. 1959, 30, 548.
Single- and Double-MoFe3S4Cubane-Type Clusters
Inorganic Chemistry, Vol. 22, No. 20, 1983
t
0.97
c
j
-800
I
,
I
L
I
-400 0 00 4 00 VELOCITY (mm/sec)
1
8.00
Figure 3. M6ssbauer spectra of polycrystalline (Et4N)JMoFe3S4(S-p-C6H4Cl),((al)gat)](6) at 4.2 K in longitudinal magnetic fields of (a) 60 kOe and (b) 80 kOe. The solid line (c) is a theoretical simulation of spectrum b assuming two magnetic subsites with intensity ratio 2:1.
spectrum of the polycrystalline Et4N+ salt, shown in Figure 2,consists of an apparent quadrupole doublet with isomer shift 6 = 0.30 f 0.03 mm/s and quadrupole splitting AEQ= 1.31 f 0.06 mm/s. However, the relatively broad line width (r = 0.56 mm/s) implies inequivalence of the Fe sites in 6. The ambient-temperature crystal structure of the compound contains a cluster that approaches C, sy”etry.11v12 In idealized C, symmetry, the highest possible for the cluster owing to the presence of the catecholate chelate ring at the Mo atom,25two Fe atoms are related by the mirror plane that bisects the ring and contains Mo, Fe, and two S atoms. The spectrum was satisfactorily fit with two quadrupole doublets of narrower line widths and constrained in a 2:l intensity ratio. The doublets in this and the 80 K spectrum differ primarily in their quadrupole splittings. In fluid acetonitrile solution at ambient temperature the ‘H N M R spectrum of 6 is consistent with C, symmetry, a property of all ligated clusters.11v’2Because the exact structure in solution is unknown, two-site fits were also investigated but the relative intensities of the doublets were left as a free parameter. Best fits of frozen-solution spectra at 80 and 130 K yielded isomer shifts essentially indistinguishable from those in the solid state but smaller quadrupole splittings. Previously we have introduced the empirical relationship (6)between isomer shift and (mean) oxidation state 6 = 1.44- 0.43s (6)
s of an Fe atom in a tetrahedral S4 site.I4 The equality of, or very small differences in, 6 values for the subsites of 6 in the solid and solution phases shows that the a oxidation level is electronically delocalized, with the mean oxidation state approximation Fe2,67+following from eq 6 . Incipient hyperfine interactions arising from the paramagnetism of the cluster result in additional line broadening and an asymmetry in the spectrum of 6 at T 5 80 K (not shown). At 4.2 K the lower energy absorption feature is broader, indicating negative signs of the principal components of the electric field gradient (efg) tensors. (25) Crystallographically imposed C, symmetry is found in the structure of the Et4N+ salt of the closely related cluster [MoFe3S4Cl3((a1)~cat)(THF)]*-.’)
0
I 20
I
I
40
60 H, ( k O e )
I
80
100
Figure 4. Total magnetic field at the nucleus H , in (Et4N),[M~Fe~S~(S-p-C~H.,Cl)~((al)~cat)] ( 6 ) as a function of the applied field Ho:(0)polycrystalline sample; (0,A) two different acetonitrile solution samples. Lines a and b refer to the less intense and more intense subsites, respectively.
Application of an external magnetic field at 4.2 K results in induced magnetic hyperfine splittings in addition to the splittings due to the applied field. Spectra at Ho= 60 and 80 kOe and 4.2 K for the polycrystalline Et4N+ salt of 6 are presented in Figure 3. At least two magnetic subsites with 2:l relative intensities are resolved. Splittings of the features of the more and less intense subsites decrease and increase, respectively, with increasing Ho.This behavior is indicative of antiparallel exchange coupling of the Fe spins in the cluster, such as has been observed in, e.g., the ferredoxin site analogues [Fe4S4(SR)4]3-?3*26,27 The more intense subsite is associated with the negative hyperfine field corresponding to the majority spins. The magnetically perturbed spectra were simulated with the assumption of two magnetic subsites. Quadrupole splittings and isomer shifts were taken from the zero-field spectral fits, and the angle between the principal component of the efg tensor and the magnetic field was averaged, as required for a powder sample. For simplicity the efg tensor was assumed to be symmetric (asymmetry parameter q = 0). Comparison of the simulated spectra with the data allows an estimate of the total magnetic field H,, for the subsites at each value of Ho.These are plotted in Figure 4 for 6 in the polycrystalline and solution states. The total field at the nucleus in subsite i is given by eq 7. If the induced spin and hence the hyperfine
an, = a0 + ahf,
(7)
field Hhf are parallel to Ho, eq 7 becomes a scalar equation and Hhf,can be determined from the intercepts of the plots in Figure 4. This procedure gives -176 and +128 kOe for the more and less intense subsites, respectively. Magnetization data for 6 in the polycrystalline state at 1.43 and 4.2 K in applied fields up to 5 5 kOe are presented in Figure 5. The saturation magnetic moment pSat 3 pB at (26) Lane, R. W.; Wcdd, A, G.; Gillum, W. 0.;Laskowski, E. J.; Holm, R. H.; Frankel, R. B.; Papaefthymiou, G. C. J . Am. Chem. Soc. 1977, 99, 2350. (27) Stephan, D. W.; Papaefthymiou, G. C.; Frankel, R. B.; Holm, R. H. Inorg. Chem. 1983, 22, 1550.
2854 Inorganic Chemistry, Vol. 22, No. 20, 1983 I
I
I
I
Mascharak et al.
I
3
i
i
0.92L
2
0 841
c
P
t $
Y
H
1-800 -400 - 2 0 0 0 0 0
200
400
800
I
Ho ( k O e )
,
Figure 5. Magnetization of polycrystalline (Et4N)3[M~Fe3S4(S:pC6H4Cl)4((al)$at)](6) at 1.43 and 4.2 K as a function of the applied field HW The solid and dashed lines are theoretical simulations with D = E = 0 and D = 3 ~ cm-I, 1 E = 0, respectively.
the lower temperature confirms the S = 3 / 2 ground state of the cluster. At 4.2 K the saturation limit is not achieved. Effective magnetic moments of 6 in the solid and solution states at ambient temperature (Table I) are slightly larger than the spin-only moment of 3.87 pBbut are consistent with a quartet ground state. All single cubanes 5 and 6 with the CY oxidation level have pcff= 3.9-4.1 pB at ambient temperature."12 The magnetization results were analyzed with use of the spin Hamiltonian (8) where D and E are the axial and rhombic
He = D[S; - j/3S(S
+ I)] + E ( S 2 - S:) + g&Ba*g
(8)
zero-field splitting parameters, g, is the electronic g value, and Reasonable simulations were achieved with E = 0 and D = 0, f l cm-' (Figure 5 ) . A more accurate value of D could not be obtained by this method because the simulations for a powder sample are not sensitive to small values of De2* The hyperfine field is related to the induced spin by eq 9,
pB is the Bohr magneton.
Hhfi =
/gn*pn
(9)
where Aoi is the magnetic hyperfine coupling constant for subsite i, ( S ) is the expectation value of the electron spin, gn* is the nuclear g value of the 14.4-keV level of 57Fe,and pLis the nuclear magneton. Because of the small value of D, ( S ) r 3/2 when Ho5 30 kOe. With use of the experimental values of HM, A. = 4.6 X 10-4 cm-I and -3.5 X lo4 cm-I for the more intense and less intense subsites, respectively. ~oFe,S4(Sp-Cd-14CI),( (al)@) (EtCN)f+ (7). The Et4N+ salt of this cluster is the only reduced single cubane thus far isolated.16 Its crystal structure has not been determined, but its IH N M R spectrum and that of its PEt, adduct are consistent with the solvated description 7 and a ligated structure analogous to 6,12respectively. The Mossbauer spectrum of the polycrystalline compound at 4.2-200 K consists of two well-resolved quadrupole doublets. The 4.2 K spectrum is shown in Figure 6 together with a theoretical least-squares fit based on two subsites. The doublets have relative intensities that are very close to 2: 1. The larger quadrupole splitting is
(3) in a preferred plane, the shapes of the magnetization curves are much more sensitive to the value of D. For an examole. cf.: Pamefthvmiou. G. C.: Frankel, R. B.; Foner, S.;Tang, S. C.; K&h, S.; Hblm, R. H. J . Phys. (Orsay, Fr.) 1976, 37, C6-209.
(28) For large axial distortions which result in the orientation of
-100
-5.0
0.0
VELOCITY
_-_____ 5.0
IO 0
(mmlsec)
Figure 6. Mossbauer spectra of polycrystalline (Et4N)3[M~Fe3S4(S-p-C,H4Cl)3((al)2cat)(EtCN)] (7) at 4.2 K and (a) Ho= 0 and (b) Ho = 80 kOe. The solid line in (a) is a theoretical least-squares fit to the data assuming two inequivalent Fe sites and based on the parameters in Table I.
a property of the less intense subsite. That the subsite isomer shifts are indistinguishable demonstrates substantial electron delocalization in the p oxidation level. From eq 6 with 6 = 0.41 mm/s it is found that s = 2.40, a value interpreted as the mean oxidation state Fe2.33+. As with 6, application of an external field at 4.2 K results in induced magnetic hyperfine splittings in addition to the splittings produced by Ho.The spectrum of 7 at Ho= 80 kOe is included in Figure 6; the overall splitting is comparable to that in the 80-kOe spectrum of 6. Clearly distinct magnetic subsites are not seen in the high-field spectrum. However, the presence of induced hyperfine fields requires a paramagnetic ground state. The ambient-temperature magnetic moments peff= 5.12 pB (solid) and 4.99 pB (solution) are consistent with a S = 2 spin system, for which the spin-only value is 4.90 pB. The only other isolated cluster with the fl oxidation level is the fully reduced double cubane [Mo2Fe6Ss(SPh),l5- (3).14 Its magnetic moment (4.84pB/subcluster) and Mossbauer parametersI4 from a two-site fit of the 4.2 K spectrum (6, = 0.42, aEQ,= 0.84, 62 = 0.41, AEQ, = 2.21 mm/s, solid state) are nearly identical with those of 7. This situation provides confirmation of an earlier argumentI4 that the -2:l intensity distribution of the doublets of 3 arises from sites within each subcluster rather than from some combination of inequivalent subclusters. While such a distribution is consistent with cluster C, symmetry, it is not evident why it should occur in the subclusters of 3, where it persists in solid and solution samples at 4.2-130 K. The ambient-temperature structure of this species involves no imposed symmetry, but each subcluster is dominantly trigonal. Structural parameters do not permit a clear sorting of Fe sites into two types. [Mo,F~~SB(S~-C~~)~((~)~C~~)ZP(4). As seen in Figure 7, the Mossbauer spectrum of the polycrystalline Et4N+salt of 4 consists of two overlapping quadrupole doublets. The spectrum was satisfactorily fit with two doublets in a -2:l intensity ratio with a 0.06 mm/s difference in isomer shifts. From the ambient-temperature structures of the triclinic and monoclinic forms of the compound? each subcluster including the catecholate chelate ring approaches C,symmetry. Consequently, it is probable that the less intense subsite is that Fe atom involved in the Fe-(SR)-Mo bridge unit. As is the case for 6, the small differences in isomer shifts reflect a
Single- and Double-MoFe3S4Cubane-Type Clusters
(c),
I
I
c_ 0.82
-4.00
v)
-2.00
5.0
40
0.00
2.00
4.00
Z
1
0.98 -
m -
a
(b)
3.0
Y
5 0.96 2.0
0.94L I
0.921 I
-
- 8 0 - 6 0 - 4 0 -20
1
-
00 2 0 4 0
Y
6 0 8 0 100
VELOCITY (mmlsec)
Figure 7. Mossbauer spectra of polycrystalline (Et4N)4[Mo2Fe6SB(S-p-C6H4Cl)6((al),Cat)z] (4) at 4.2 K and (a) Ho 3 O and (b) Ho = 80 kOe. The solid line in (a) is a theoretical least-squares fit to the data assuming two inequivalent Fe sites and based on the parameters in Table I. delocalized structure of the CY oxidation level and the average 6 value of 0.30 mm/s indicates the mean oxidation state Fe267+. Application of an external magnetic field results in a Mossbauer spectrum of 4 that is very different from that of 6. The 80-kOe spectrum in Figure 7 (compare with Figure 3) originates from two magnetic subsites with -2:l relative intensity. The splitting of the two lines of the more intense subsite is nearly field invariant whereas that of the less intense subsite increases with increasing field. Together with magnetization results (vide infra) the spectrum is interpreted in terms of antiparallel coupling of the net spins of the two CY subclusters to produce a singlet ground state and low-lying paramagnetic state@). The states are mixed by the applied field to give a Van Vleck type paramagnetic moment which increases with increasing field. From spectra at different Ho values it is estimated that for the more intense subsite Hhf= -Ho, resulting in H , = 0. For the less intense subsite the hyperfine field is smaller and oppositely signed, i.e., HM = uHo ( a OS), giving H , = (1 u)Ho. This implies antiparallel spin coupling between Fe atoms in each subcluster. The magnetization behavior of 4 in the solid state at 4.2 K, displayed in Figure 8, is essentially linear with Houp to about 40 kOe. At 40 5 Ho5 90 kOe positive deviations from extrapolation of the linear region are observed. The moment at constant field is temperature independent below 4.2 K but increases at higher temperatures, with a maximum between 6 and 10 K (not shown). These results are indicative of a singlet ground state and an excited paramagnetic state. The increase in moment above -40 kOe at 4.2 K signals the approach (and population) of components of the excited state (possibly S = 1) to the singlet level at these high fields. For purpose of comparison the magnetization of polycrystalline (Et4N)3[MO2Fe6Ss(S-p-C6H4C1)9], a triply bridged double cubane of type 1, is included in Figure 8. Unlike the doubly bridged double cubane 4 the moment approaches saturation at Ho c 80 kOe. The saturation moment is clearly very close to 6 pB/anion or 3 pB/subcluster, in agreement with magnetic susceptibility results near 300 K for other [Mo2Fe6S8(SR),]3- and [M o 2 F e 6 ~ ( o M e ) 3 ( s R )3-6 ]clusters, J~ which accord with a net spin S = l2per s ~ b c l u s t e r . ' ~The value of 3 pBis the same as the saturation moment of the single cubane 6, an expected result since both species have the a
=
+
I.a
.. . 0
0
.
10
20
30
40
50
60
70
80
H, (kOe)
Figure 8. Magnetization per cluster anion of polycrystalline (a) (Et4N),[MoFeJS4(S-p-C6H4C1)4((al)Zcat)l (61, (b) (Et4N14[Mo2FesS~(S-p-C6H4C1)6((al)2cat)21 (41, and (C) (EtdW3[ M O ~ F ~ ~ S B ( S - ~ - C(1) ~ Has~aCfunction ~ ) ~ ] of applied magnetic field at 4.2 K.
oxidation level. Inasmuch as the subclusters of 4 also have this oxidation level, the major electronic difference between doubly bridged 4 and triply bridged 1 is the strength of the exchange coupling between subclusters. This interaction clearly is larger in 4. Summary. A substantial body of information on the electronic properties of MoFe3S4clusters of the single-8-'2 and d o u b l e - ~ u b a n e ~ types ~ ~ ~ has ~ J ~now - ~ been ~ ~ ~collected. ~ The following is a summary of the status of interpretation of certain leading electronic features of these clusters. (1) Spin Ground States. Low-temperature magnetization data are conclusive in establishing the S = 3 / 2 state for [M~Fe,S~(S-p-C~H,Cl),((al)~cat)] 3- (6) and the subclusters of [MO~F~~S~(S-~-C~H,C~)~]~(1). Ambient-temperature magnetic moments of these and other clusters of types 1,6J4,17*29 5,9310and 611J2are consistent with a quartet state as are the EPR spectra of 5 and 6 (vide infra). It is probable that this state applies to all clusters with the a ([MoFe3S4I3+)oxidation level, including the subclusters of [Mo2Fe6S8(S-p-C6H4C1),((a1)2cat)2]4- (4). In this doubly bridged double-cubane subcluster net spins appear to be more strongly coupled than in the triply bridged double cubanes 117 (Figure 8), resulting in a S = 0 ground state. One-electron reduction of CY clusters to the /3 ([MoFe3S4l2+)oxidation level, as in series 4 and 5, has thus far afforded two isolable species, [M0zFe6S8(SPh)gl5(3)14 and [MoF~~S~(S-~-C~H~C~)~((~~),C~~)(E~ (7), whose ambient-temperature magnetic moments are consistent with S = 2 states. Magnetization data for ,6 clusters are not available. (2) Charge Distribution. The isomer shift data for clusters 4 and 6 (Table I), when compared with those for clusters 1 and demonstrate that mean 6 values of a clusters are not significantly dependent on bridge structure or its presence. The most important result for these (29) Christou, G.; Collison, D.; Garner, C. D.; Mabbs, F. E.; Petrouleas, V. Inorg. Nucl. Chem. Lett. 1981, 17, 137.
2856 Inorganic Chemistry, Vol. 22, No. 20, 1983
Mascharak et al.
clusters is that isomer shifts are sensibly consistent only with mean fractional Fe oxidation states; i.e., the Fe, cluster portions are electronically delocalized to about the same extent independent of different structural features at the Mo site. Reduction to the /3 oxidation level affords an increase in mean 6 values of 3 and 7 to 0.41 mm/s in the solid state at 4.2 K. These changes of 0.10-0.1 1 mm/s may be compared with 0.14 mm/s calculated from eq 6 if the added electron density is assumed to be equally divided over the three Fe sites. This comparison provides reasonable evidence that the Fe, portions of the a and /3 clusters are those primarily affected by the redox process. Potentials of the steps in series 414J5and 59 parallel those of [Fe4S,(SR),I2-s3- couples (R constant), s u p porting the qualitative view that orbitals involved in electron gain and loss are heavily Fe-S in character. A subsidiary issue is the exact description of the mean Fe oxidation state in a and /3 clusters. For the former we have favored Fe2.67+2314918 (i, 2Fe3+ Fe2+ Mo3+),on the basis of eq 6, which provides the only means of estimating, directly from experimental data, oxidation states of Fe atoms in sites structurally similar to those in MoFe3S4clusters. Because of the empirical nature of the approach we do not insist upon the stated description, the alternative to which is Fe2.33+(ii, Fe3+ 2Fe2+ Mo4+). Magnetic Mossbauer spectra confirm antiparallel spin coupling between two Fe subsites, and the net S = 3/2 spin is derivable with use of either description. Given that the principal locus of reduction is the Fe3 portion, the isomer shifts of clusters appear to accord somewhat less well with the reduced version of state ii (3Fe2+ Mo4+) than of state i (Fe2,33+).Isomer shifts of tetrahedral Fe"S4 units at 4.2 K tend to be 0.5-0.6 mm/s3*,, and are usually at the high end of the range.34 A recent theoretical analysis of the bulk susceptibilities of triply bridged double cubanes containing a clusters did not permit a distinction between subcluster coupling schemes based on states i and ii.17 (3) Isotropic NMR Shifts. 'Hand some 19FN M R spectra have been reported for examples of clusters 1-339436J4and 5-7.8-13 The existence of dominant hyperfine contact interactions at the Fe sites effected by ligand metal antiparallel spin delocalization is supported by relative signs of arenethiolate proton, F, CH,, and CF3 shifts, near-coincidence of the temperature dependence of normalized shifts and susceptibilities of a clusters, and increased shifts upon passing from a ( S = 3/2) to the more paramagnetic /3 ( S = 2) clusters. These properties as diagnostic of contact shifts are discussed elsewhere.36 The last property is represented by eq 10, in
+
+
5 93
I
1
U
+
+
+
-
(AH/HO)~~A " ,P~ =(AH/Ho)BWn A@$
(10)
Simopoulos, A.; Papaefthymiou, V.; Kostikas, A.; Petrouleas, V.; Coucouvanis, D.; Simhon, E. D.; Stremple, P. Chem. Phys. Lett. 1981, ai, 261. Coucouvanis, D.; Stremple, P.; Simhon, E. D.; Swenson, D.; Baenziger, N. C.; Draganjac, M.; Chan, L. T.; Simopoulos, A.; Papacfthymiou, V.; Kostikas, A.; Petrouleas, V. Inorg. Chem. 1983, 22, 293. Mascharak, P. K.; Papaefthymiou, G . C.; Frankel, R. B.; Holm, R. H. J . Am. Chem. Soc. 1981,103,6110. Bearwood, P.; Gibson, J. F.; Johnson, C. E.; Rush, J. D. J . Chem. Soc., Dalton Trans. 1982, 2015. The correlation of eq 6 is based on data for well-defined Fe-S compounds, and as already stated,I4 it may be perturbed by the presence of a Mo atom within bonding distance. A possible case in point is the nominal Fe(I1)-Mo(V1) dimer [(PhS) FeS M 2- whose isomer shift of 0.32-0.33 mm/s at 4.2-77 K " - " ~ ~ ~ g i v A from eq 6 . Difficulties with this interpretation have been noted.35 While it seems unlikely that Mo(II1 or IV) at a comparable distance (-2.7 A) from three Fe atoms could cause all three to appear significantly more oxidized than Fez+, were this the most physically meaningful oxidation-state designation, the matter remains open. Tieckelmann, R. H.; Silvis, H. C.; Kent, T. A.; Huynh, B. H.; Waszczak, J. V.; Teo, B.-K.; Averill, B. A. J . Am. Chem. SOC.1980,
"%+'
102, 5550.
FeMo-co
!/ I
1,1 v
v
2 05
3 34
Figure 9. EPR spectra (X band) of FeMo-co (1.4 mM Mo) from Azotobacter uinelandii Fe protein in N-methylformamide and 10 mM [MFe~S4(S-p-C6H4C1),((al)2cat)CN]* (M = Mo, W) in acetonitrile at -7 K. All spectra were recorded at 5-mW microwave power; 10-G
and 40-G modulation amplitudes were used in the upper spectrum and lower two spectra, respectively. Selected g values are indicated.
which the A values are coupling constants of a given nucleus in the two oxidation levels. In acetonitrile at 297 K the isotropic shifts and squared magnetic moment ratios are 1.52 and 1.50, respectively, for [M~Fe~S~(S-p-C~H~Cl)~((al)~cat)(MeCN)I2- (5) and 7.37 Thus the ratio &/A, z 1, a reasonable result for two species with pure contact shifts produced by the same delocalization mechanism. Isotropic shifts of Mo-bound ligands are also found in the clusters 5 and 6. Those of L = arenethiolates are very small (51 ppm), but those of catecholate ring protons can be as large as -+5 ppm and vary from -3 to +4 ppm in CY and /3 clusters, respectively, at ambient temperature. Shifts in both oxidation levels vary with solvent and ligand L.9J0J2The contact and dipolar contributions to these shifts are unknown. (4) EPR Spectra. Spectra of double cubanes of types 117 and 48 in the solid state are complex, presumably because of weak subcluster coupling. The powder spectrum of [W2Fe6S8(OMe)3(SPh)6]3as well as the temperature dependence of its magnetic susceptibility are best simulated by using a theoretical model including such an intera~ti0n.l~ The spectra of the single cubanes 5 and 6, several examples of which have been presented 1*12 are substantially simpler and are consistent with a S = 3/2 spin system with a small rhombic distortion in both solid and solution phases. The solution spectrum of [MoF~~S~(S-~-C,H~C~),((~~),~~~ (6, L = CN-;4.09 pB at 295 K), shown in Figure 9 with that of its tungsten analogue, is typical. Spectra of single cubanes are best observed at T 5 15 K because of rapid electron spin relaxation. For example, the peak height of the g = 4.4 feature of 10 mM [M~Fe~s,(sPh)~(Pr~cat)(Me~SO)]~in M e 2 S 0 (36) Reynolds, J. G.;Laskowski, E. J.; Holm, R. H. J. Am. Chem. Soc. 1978, 100, 531 5 and references therein. (37) Isotropic shifts of m-H: -9.17 (I),-6.04(5) ppm. Magnetic moments (Ned: 4.99 (7). 4.08 (5) WE.
Inorganic Chemistry, Vol. 22, No. 20, 1983 2857
Single- and Double-MoFe3S4Cubane-Type Clusters
Table 11. Comparative Electronic Properties of Synthetic and Native M o - F e S Clusters property
synthetic
ground state S sav, "/sd
A , , cm-' g values
D cm-' Af hf broadening 9sM~ 57Fe(g FC: 2) (AH/Ho)iso (19F, R =p-C,H,F, P-C, H4CF 3 ) e
(a),2
3/z,
none small (-8%) negative
a Refers t o FeMo-co unless otherwise noted. Unresolved. A = 2D(1 3A)1'2.38
+
nativea
(PI
> 1 (integer, red. formb) 0.28-0.32 ( 0 ) 0.25, 0.28-0.2gb 0.40-0.42 (0) 0.34 (0.24) 4.6 x 10-4, -3.5 x 10-4 (a) (3.7-5.6) X -3.1 x 10-4 4.4-4.6, -3.5: S2.01*'/2), -5.91*3/z) (a) 4.6, 3.
